davoid

This suggests that the PPLO elementary bodies do work to retain their hydrogen ions, that they somehow do not depend on hydrogen ions, or that most small PPLO cells do not survive. Which of these is correct is at present not clear. The essential point in Schrödinger's discussion is that cells must be large enough so that statistical errors are not lifethreatening. Cells cannot afford to have just a few copies of essential atoms or molecules unless there are also mechanisms to correct errors. One copy of each gene is protected by a stable means of reproduction, of error correction, and of precise distribution during fission. Also, genes can be transcribed and translated into multiple copies. The provision for copies is an important safeguard against error, since it ensures that at least one good copy of the needed enzyme is present in each cell, a copy that can be used many times to catalyze reactions.

From these considerations we see that PPLO cells, in particular, and prokaryotes, in general, are close to the lower limit of viable cells. The actual lower limit will depend on nutritional requirements, stability of genetic information storage, and accuracy in expressing that information so as to ensure that needed copies of functional proteins are produced. Minimal information needs and the mechanisms needed to assure essential redundancy are thus balanced.

From the foregoing discussion, we realize that the first organisms could not have been very small, since, with a limited ability to form informed macromolecules, many errors would be probable and a high redundancy would be needed to compensate for the effects of those errors. And then, thinking of eukaryotes, to which we turn next, their relatively large size ensures that these cells can do more than just compound a limited amount of information (redundancy); they are large enough to carry a variety of information. We expect, in fact, we predict the eukaryotes to be truly complex.

Returning to the problem of the needs of the smallest cell, we examine that problem from another point of view. This comes from considerations raised by the great physicist Erwin Schrödinger, who was also a profound student of molecular biology. He pointed out that small size has a very important implication in terms of the errors that are permissible. Statistically, errors are expressed as standard errors and a general estimate of a standard error is plus or minus the square root of some average value. If n is the average value, the standard error is ±. Now, as Schrödinger points out, when n is large, for example, 6 x 1023, the standard error is proportionately small. This consideration can be applied to the occurrence of atoms of a certain type in a certain space. Compared to 6 x 1023, the error of 8 x 1011 is a fraction of a percent. But when n is smaller the standard error is proportionately larger.

Now let us look at the situation involving hydrogen ions. The concentration of these atoms determines the acidity of a solution. Most bacteria live under conditions where the hydrogen ion concentration is 10-7 grams per liter, or pH 7. If the pH is the same inside and outside the cell (hydrogen ions are small and can move freely across a membrane; work must be done to maintain different concentrations on either side of a membrane) then in a coccoidal bacterium 0.5 μm in diameter there would be only about 3.6 hydrogen ions. With an n this low, statistically, a significant number of the cells will have no hydrogen ions. (According to the Poisson distribution we should expect, on the average, that this will be 0.027 or about 3% of the cells.) The absence of these ions will have a profound effect on the chemistry of these cells. If the size of the cell decreases, the probability of the absence of hydrogen ions increases dramatically. In a cell with a diameter of 0.1 μm, the possibility of a hydrogen ion being present is 0.03; thus, most cells will have no hydrogen ion.

The trend away from state-supported political terrorism and toward more diverse, free-wheeling, transnational networks—enabled by information technology—will continue. Some of the states that actively sponsor terrorism or terrorist groups today may decrease or even cease their support by 2015 as a result of regime changes, rapprochement with neighbors, or the conclusion that terrorism has become counterproductive. But weak states also could drift toward cooperation with terrorists, creating defacto new state supporters…. Between now and 2015 terrorist tactics will become increasingly sophisticated and designed to achieve mass casualties. We expect the trend toward greater lethality in terrorist attacks to continue.

Many potential adversaries, as reflected in doctrinal writings and statements, see U.S. military concepts, together with technology, as giving the United States the ability to expand its lead in conventional war-fighting capabilities. This perception among present and potential adversaries will continue to generate the pursuit of asymmetric capabilities against U.S. forces and interests abroad as well as the territory of the United States. U.S. opponents—state and such non-state actors as drug lords, terrorists, and foreign insurgents—will not want to engage the U.S. military on its terms. They will choose instead political and military strategies designed to dissuade the United States from using force, or, if the United States does use force, to exhaust American will, circumvent or minimize U.S. strengths, and exploit perceived U.S. weaknesses. Asymmetric challenges can arise across the spectrum of conflict that will confront U.S. forces in a theater of operations or on U.S. soil.

Much of the terrorism noted earlier will be directed at the United States and its overseas interests. Most anti–U.S. terrorism will be based on perceived ethnic, religious, or cultural grievances. Terrorist groups will continue to find ways to attack U.S. military and diplomatic facilities abroad. Such attacks are likely to expand increasingly to include U.S. companies and American citizens. Middle East and Southwest Asian-based terrorists are the most likely to threaten the United States.